Figure S1. Thermogravimetric analysis (black line) and differential thermal analysis (blue line) of Au nanoparticles coated with oleylamine. The feature in the differential thermal analysis at 165°C, and the drop in weight at the same temperature, indicate the initial removal of the oleylamine surfactant.Thermal analysis was performed using an Alphatech SDT Q600 thermoanalyser under flowing air (50 mL/min) at a heating rate of 10 °C/min.

Achieving stability with highly active Ru nanoparticles for electrocatalysis is a major challenge for the oxygen evolution reaction. As improved stability of Ru catalysts has been shown for bulk surfaces with low-index facets, there is an opportunity to incorporate these stable facets into Ru nanoparticles. Now, a new solution synthesis is presented in which hexagonal close-packed structured Ru is grown on Au to form nanoparticles with 3D branches. Exposing low-index facets on these 3D branches creates stable reaction kinetics to achieve high activity and the highest stability observed for Ru nanoparticle oxygen evolution reaction catalysts. These design principles provide a synthetic strategy to achieve stable and active electrocatalysts.

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